Ocean Thermal Energy Conversion Cold Water Pipe

ABSTRACT

An offshore power generation structure comprising a submerged portion having heat exchange sections, power generation sections, a cold water pipe and a cold water pipe connection. The cold water pipe comprises a plurality of offset first and second staved portions.

TECHNICAL FIELD

This invention relates to ocean thermal energy conversion power plantsand more specifically to floating, minimum heave platform, multi-stageheat engine, ocean thermal energy conversion power plants.

BACKGROUND

Energy consumption and demand throughout the world has grown at anexponential rate. This demand is expected to continue to rise,particularly in developing countries in Asia and Latin America. At thesame time, traditional sources of energy, namely fossil fuels, are beingdepleted at an accelerating rate and the cost of exploiting fossil fuelscontinues to rise. Environmental and regulatory concerns areexacerbating that problem.

Solar-related renewable energy is one alternative energy source that mayprovide a portion of the solution to the growing demand for energy.Solar-related renewable energy is appealing because, unlike fossilfuels, uranium, or even thermal “green” energy, there are few or noclimatic risks associated with its use. In addition, solar relatedenergy is free and vastly abundant.

Ocean Thermal Energy Conversion (“OTEC”) is a manner of producingrenewable energy using solar energy stored as heat in the oceans'tropical regions. Tropical oceans and seas around the world offer aunique renewable energy resource. In many tropical areas (betweenapproximately 20° north and 20° south latitude) the temperature of thesurface sea water remains nearly constant. To depths of approximately100 ft the average surface temperature of the sea water variesseasonally between 75° F. and 85° F. or more. In the same regions, deepocean water (between 2500 ft and 4200 ft or more) remains a fairlyconstant 40° F. Thus, the tropical ocean structure offers a large warmwater reservoir at the surface and a large cold water reservoir atdepth, with a temperature difference between the warm and coldreservoirs of between 35° F. to 45° F. This temperature differenceremains fairly constant throughout the day and night, with smallseasonal changes.

The OTEC process uses the temperature difference between surface anddeep sea tropical waters to drive a heat engine to produce electricalenergy. OTEC power generation was identified in the late 1970's as apossible renewable energy source having a low to zero carbon footprintfor the energy produced. An OTEC power plant, however, has a lowthermodynamic efficiency compared to more traditional, high pressure,high temperature power generation plants. For example, using the averageocean surface temperatures between 80° F. and 85° F. and a constant deepwater temperature of 40° F., the maximum ideal Carnot efficiency of anOTEC power plant will be 7.5 to 8%. In practical operation, the grosspower efficiency of an OTEC power system has been estimated to be abouthalf the Carnot limit, or approximately 3.5 to 4.0%. Additionally,analysis performed by leading investigators in the 1970's and 1980's,and documented in “Renewable Energy from the Ocean, a Guide to OTEC”William Avery and Chih Wu, Oxford University Press, 1994 (incorporatedherein by reference), indicates that between one quarter to one half (ormore) of the gross electrical power generated by an OTEC plant operatingwith a ΔT of 40° F. would be required to run the water and working fluidpumps and to supply power to other auxiliary needs of the plant. On thisbasis, the low overall net efficiency of an OTEC power plant convertingthe thermal energy stored in the ocean surface waters to net electricenergy has not been a commercially viable energy production option.

An additional factor resulting in further reductions in overallthermodynamic efficiency is the loss associated with providing necessarycontrols on the turbine for precise frequency regulation. Thisintroduces pressure losses in the turbine cycle that limit the work thatcan be extracted from the warm sea water. The resulting net plantefficiency would then be between 1.5% and 2.0%.

This low OTEC net efficiency compared with efficiencies typical of heatengines that operate at high temperatures and pressures has led to thewidely held assumption by energy planners that OTEC power is too costlyto compete with more traditional methods of power production.

Indeed, the parasitic electrical power requirements are particularlyimportant in an OTEC power plant because of the relatively smalltemperature difference between the hot and cold water. To achievemaximum heat transfer between the warm sea water and the working fluid,and between the cold sea water and the working fluid large heat exchangesurface areas are required, along with high fluid velocities. Increasingany one of these factors can significantly increases the parasitic loadon the OTEC plant, thereby decreasing net efficiency. An efficient heattransfer system that maximizes the energy transfer in the limitedtemperature differential between the sea water and the working fluidwould increase the commercial viability of an OTEC power plant.

In addition to the relatively low efficiencies with seemingly inherentlarge parasitic loads, the operating environment of OTEC plants presentsdesign and operating challenges that also decrease the commercialviability of such operations. As previously mentioned, the warm waterneeded for the OTEC heat engine is found at the surface of the ocean, toa depth of 100 ft or less. The constant source of cold water for coolingthe OTEC engine is found at a depth of between 2700 ft and 4200 ft ormore. Such depths are not typically found in close proximity topopulation centers or even land masses. An offshore power plant isrequired.

Whether the plant is floating or fixed to an underwater feature, a longcold water intake pipe of 2000 ft or longer is required. Moreover,because of the large volume of water required in commercially viableOTEC operations, the cold water intake pipe requires a large diameter(typically between 6 and 35 feet or more). Suspending a large diameterpipe from an offshore structure presents stability, connection andconstruction challenges which have previously driven OTEC costs beyondcommercial viability.

Additionally, a pipe having significant length to diameter ratio that issuspended in a dynamic ocean environment can be subjected to temperaturedifferences and varying ocean currents along the length of the pipe.Stresses from bending and vortex shedding along the pipe also presentchallenges. And surface influences such as wave action present furtherchallenges with the connection between the pipe and floating platform.

An enormous challenge of OTEC operations has been the need to fullyassemble a pipe having a length of 2000 ft to 4000 ft or more andtransporting such a pipe to the operational site. Furthermore, a greaterchallenge has been upending such a pipe for installation to a floatingplatform and ultimately making a connection to the platform.

Previous OTEC cold water pipe construction has used segmented pipes. Asegmented pipe is a pipe constructed of cylindrical segments joinedtogether in series to the obtain the desired length. Segmented pipes aredisclosed in “Ocean Thermal Energy Conversion Cold Water PipePreliminary Design Project,” TRW Energy Systems Group, Final Report,Nov. 20, 1979 (incorporated by reference herein in its entirety).Segmented pipes can be heavier and less flexible than continuouslyconstructed pipes. Moreover, various connection methods can interferewith the flow of the fluid through the pipe.

A cold water pipe intake system having desirable construction,installation and performance criteria would increase the viability of anOTEC power plant.

SUMMARY

Aspects of the present invention are directed to a power generationplant utilizing ocean thermal energy conversion processes.

Further Aspects of the invention relate to an offshore OTEC power planthaving improved overall efficiencies with reduced parasitic loads,greater stability, lower construction and operating costs, and improvedenvironmental footprint. Other aspects include large volume waterconduits that are integral with the floating structure. Modularity andcompartmentation of the multi-stage OTEC heat engine reducesconstruction and maintenance costs, limits off-grid operation andimproves operating performance. Still further aspects provide for afloating platform having integrated heat exchange compartments andprovides for minimal movement of the platform due to wave action. Theintegrated floating platform may also provide for efficient flow of thewarm water or cool water through the multi-stage heat exchanger,increasing efficiency and reducing the parasitic power demand. Aspectsof the invention can promote an environmentally neutral thermalfootprint by discharging warm and cold water at appropriatedepth/temperature ranges. Energy extracted in the form of electricityreduces the bulk temperature to the ocean.

Still further aspects of the invention relate to a cold water pipe foruse with an offshore OTEC facility, the cold water pipe being an offsetstaved, continuous pipe.

An aspect relates to a pipe that comprises an elongate tubular structurehaving an outer surface, a top end and a bottom end. The tubularstructure comprises a plurality of first and second staved segments,each stave segment has a top portion and a bottom portion, wherein thetop portion of the second stave segment is offset from the top portionof the first staved segment.

A further aspect relates to a pipe comprising a ribbon or a strake atleast partially wound around the pipe on the outside surface of thetubular structure. The ribbon or strake can be circumferentially woundaround the outer surface of the top portion of the pipe, the middleportion of the pipe, or the lower portion of the pipe. The ribbon orstrake can be circumferentially wound around the entire length of thepipe. The ribbon or strake can be a be attached so as to laysubstantially flat against the outer surface of the pipe. The ribbon orstrake can be attached so as to protrude outwardly from the outersurface of the pipe. The ribbon or strake can be made of the same ordifferent material as the pipe. The ribbon or strake can be adhesivelybonded to the outer surface of the pipe, mechanically bounded to theouter surface of the pipe, or use a combination of mechanical andadhesive bonds to attach to the outer surface of the pipe.

Further aspects of the invention relate to an offset staved pipe whereineach stave segment further comprises a tongue on a first side and agroove on a second side for mating engagement with an adjacent stavesegment. The offset stave pipe can include a positive locking system tomechanically couple a first side of one stave to the second side of asecond stave. Stave can be joined vertically from the top portion of onestave to the bottom portion of an adjacent stave using biscuit joinery.In an alternative embodiment, the top portion of a stave and the bottomportion of a stave can each include a joining void, such that when thetop portion of a first stave is joined with the bottom portion of asecond stave, the joining voids align. A flexible resign can be injectedinto the aligned joining voids. The flexible resign can be used to fillgaps in any joined surfaces. In aspects of the invention the flexibleresign is a methacrylate adhesive.

Individual staves of the current invention can be of any length. Inaspects each stave segment is between 20 feet and 90 feet measured fromthe bottom portion to the top portion of the stave. Stave segments canbe sized to be shipped by standard inter-modal container. Individualstave segments can be between 10 inches and 120 inches wide. Each stavesegment can be between 1 inch and 24 inches thick.

In aspects of the invention stave segments can be pultruded, extruded,or molded. Stave segments can comprise polyvinyl chloride (PVC),chlorinated polyvinyl chloride (CPVC), fiber reinforced plastic (FRP),reinforced polymer mortar (RPMP), polypropylene (PP), polyethylene (PE),cross-linked high-density polyethylene (PEX), polybutylene (PB),acrylonitrile butadiene styrene (ABS); polyester, fiber reinforcedpolyester, nylon reinforced polyester, vinyl ester, fiber reinforcedvinyl ester, nylon reinforced vinyl ester, concrete, ceramic, or acomposite of one or more thereof.

In further aspects of the invention the materials selected can provideneutral buoyancy of the fully assembled pipe.

In further aspects of the invention, a stave segment can comprise atleast one internal void. At least one void can be filled with water,resin, adhesive, polycarbonate foam, or syntactic foam.

In aspects of the invention, the pipe is a cold water intake pipe for anOTEC power plant.

A still further aspect of the invention relates to an offshore powergeneration structure comprising a submerged portion, the submergedportion further comprises: a heat exchange portion; a power generationportion; and a cold water pipe comprising a plurality of offset firstand second stave segments.

Yet another aspect of the invention relates to a method of forming acold water pipe for use in an OTEC power plant, the method comprises:forming a plurality of first and second stave segments joiningalternating first and second stave segments such that the second stavesegments are offset from the first stave segments to form a continuouselongated tube.

A further aspect of the invention relates to a submerged vertical pipeconnection comprising: a floating structure having a vertical pipereceiving bay, wherein the receiving bay has a first diameter; avertical pipe for insertion into the pipe receiving bay, the verticalpipe having a second diameter smaller than the first diameter of thepipe receiving bay; a partially spherical or arcuate bearing surface;and one or more movable detents, pinions or lugs operable with thebearing surface, wherein the detents define a diameter that is differentthan the first or second diameter when in contact with the bearingsurface.

An additional aspect of the invention relates to a method of connectinga submerged vertical pipe to a floating platform comprising: providing afloating structure having a vertical pipe receiving bay, wherein thepipe receiving bay has a first diameter, providing a vertical pipehaving a top end portion that has a second diameter that is less thanthe first diameter; inserting the top end portion of the vertical pipeinto the receiving bay; providing a bearing surface for supporting thevertical pipe; extending one or more detents such that the one or moredetents have a diameter that is different from the first or seconddiameters; contacting the one or more detents with the bearing surfaceto suspend the vertical pipe from the floating structure.

Aspects of the invention may have one or more of the followingadvantages: a continuous offset staved cold water pipe is lighter thansegmented pipe construction; a continuous offset staved cold water pipehas less frictional losses than a segmented pipe; individual staves canbe sized to for easy transportation to the OTEC plant operational site;staves can be constructed to desired buoyancy characteristics; massproduced uniform parts (i.e., staves) are ultimately cheaper and providequality control assurance than single unitary pipe (i.e. spiral woundpipes); OTEC power production requires little to no fuel costs forenergy production; the low pressures and low temperatures involved inthe OTEC heat engine reduce component costs and require ordinarymaterials compared to the high-cost, exotic materials used in highpressure, high temperature power generation plants; plant reliability iscomparable to commercial refrigeration systems, operating continuouslyfor several years without significant maintenance; reduced constructiontimes compared to high pressure, high temperature plants; and safe,environmentally benign operation and power production. Additionaladvantages may include, increased net efficiency compared to traditionalOTEC systems, lower sacrificial electrical loads; reduced pressure lossin warm and cold water passages; modular components; less frequentoff-grid production time; minimal heave and reduced susceptibility towave action; discharge of cooling water below surface levels, intake ofwarm water free from interference from cold water discharge.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an exemplary prior-art OTEC heat engine.

FIG. 2 illustrates an exemplary prior-art OTEC power plant.

FIG. 3 illustrates OTEC structure of the present invention.

FIG. 4 illustrates an offset staved pipe of an OTEC structure of thepresent invention.

FIG. 5 illustrates a detailed image of an offset stave pattern of thepresent invention.

FIG. 6 illustrates a cross sectional view of an offset staved cold waterpipe of the present invention.

FIGS. 7A-C illustrate various views of individuals staves of the presentinvention.

FIG. 8 illustrates a tongue and groove arrangement of an individualstave of the present invention.

FIG. 9 illustrates a positive snap lock between two staves of thepresent invention.

FIG. 10 illustrates an offset staved cold water pipe incorporating areinforcing strake of the present invention.

FIG. 11 illustrates a method of cold water pipe construction of thepresent invention.

FIG. 12 illustrates an exemplary OTEC heat engine of the presentinvention.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This invention relates to electrical power generation using OceanThermal Energy Conversion (OTEC) technology. Aspects of the inventionrelate to a floating OTEC power plant having improved overallefficiencies with reduced parasitic loads, greater stability, lowerconstruction and operating costs, and improved environmental footprintover convention OTEC power plants. Other aspects include large volumewater conduits that are integral with the floating structure. Modularityand compartmentation of the multi-stage OTEC heat engine reducesconstruction and maintenance costs, limits off-grid operation andimproves operating performance. Still further aspects provide for afloating platform having integrated heat exchange compartments andprovides for minimal movement of the platform due to wave action. Theintegrated floating platform may also provide for efficient flow of thewarm water or cool water through the multi-stage heat exchanger,increasing efficiency and reducing the parasitic power demand. Aspectsof the invention promote a neutral thermal footprint by discharging warmand cold water at appropriate depth/temperature ranges. Energy extractedin the form of electricity reduces the bulk temperature to the ocean.

OTEC is a process that uses heat energy from the sun that is stored inthe Earth's oceans to generate electricity. OTEC utilizes thetemperature difference between the warmer, top layer of the ocean andthe colder, deep ocean water. Typically this difference is at least 36°F. (20 C). These conditions exist in tropical areas, roughly between theTropic of Capricorn and the Tropic of Cancer, or even 20° north andsouth latitude. The OTEC process uses the temperature difference topower a Rankine cycle, with the warm surface water serving as the heatsource and the cold deep water serving as the heat sink. Rankine cycleturbines drive generators which produce electrical power.

FIG. 1 illustrates a typical OTEC Rankine cycle heat engine 10 whichincludes warm sea water inlet 12, evaporator 14, warm sea water outlet15, turbine 16, cold sea water inlet 18, condenser 20, cold sea wateroutlet 21, working fluid conduit 22 and working fluid pump 24.

In operation, heat engine 10 can use any one of a number of workingfluids, for example commercial refrigerants such as ammonia. Otherworking fluids can include propylene, butane, R-22 and R-134a. Othercommercial refrigerants can be used. Warm sea water betweenapproximately 75° F. and 85° F., or more, is drawn from the oceansurface or just below the ocean surface through warm sea water inlet 12and in turn warms the ammonia working fluid passing through evaporator14. The ammonia boils to a vapor pressure of approximately 9.3 atm. Thevapor is carried along working fluid conduit 22 to turbine 16. Theammonia vapor expands as it passes through the turbine 16, producingpower to drive an electric generator 25. The ammonia vapor then enterscondenser 20 where it is cooled to a liquid by cold sea water drawn froma deep ocean depth of approximately 3000 ft. The cold sea water entersthe condenser at a temperature of approximately 40° F. The vaporpressure of the ammonia working fluid at the temperature in thecondenser 20, approximately 51° F., is 6.1 atm. Thus, a significantpressure difference is available to drive the turbine 16 and generateelectric power. As the ammonia working fluid condenses, the liquidworking fluid is pumped back into the evaporator 14 by working fluidpump 24 via working fluid conduit 22.

The heat engine 10 of FIG. 1 is essentially the same as the Rankinecycle of most steam turbines, except that OTEC differs by usingdifferent working fluids and lower temperatures and pressures. The heatengine 10 of the FIG. 1 is also similar to commercial refrigerationplants, except that the OTEC cycle is run in the opposite direction sothat a heat source (e.g., warm ocean water) and a cold heat sink (e.g.,deep ocean water) are used to produce electric power.

FIG. 2 illustrates the typical components of a floating OTEC facility200, which include: the vessel or platform 210, warm sea water inlet212, warm water pump 213, evaporator 214, warm sea water outlet 215,turbine-generator 216, cold water pipe 217, cold sea water inlet 218,cold water pump 219, condenser 220, cold sea water outlet 221, workingfluid conduit 22, working fluid pump 224, and pipe connections 230. OTECfacility 200 can also include electrical generation, transformation andtransmission systems, position control systems such as propulsion,thrusters, or mooring systems, as well as various auxiliary and supportsystems (for example, personnel accommodations, emergency power, potablewater, black and grey water, fire fighting, damage control, reservebuoyancy, and other common shipboard or marine systems).

Implementations of OTEC power plants utilizing the basic heat engine andsystem of FIGS. 1 and 2 have a relatively low overall efficiency of 3%or below. Because of this low thermal efficiency, OTEC operationsrequire the flow of large amounts of water through the power system perkilowatt of power generated. This in turn requires large heat exchangershaving large heat exchange surface areas in the evaporator andcondensers.

Such large volumes of water and large surface areas require considerablepumping capacity in the warm water pump 213 and cold water pump 219,reducing the net electrical power available for distribution to ashore-based facility or on board industrial purposes. Moreover, thelimited space of most surface vessels, does not easily facilitate largevolumes of water directed to and flowing through the evaporator orcondenser. Indeed, large volumes of water require large diameter pipesand conduits. Putting such structures in limited space requires multiplebends to accommodate other machinery. And the limited space of typicalsurface vessels or structures does not easily facilitate the large heatexchange surface area required for maximum efficiency in an OTEC plant.Thus the OTEC systems and vessel or platform have traditional been largeand costly. This has lead to an industry conclusion that OTEC operationsare a high cost, low yield energy production option when compared toother energy production options using higher temperatures and pressures.

Aspects of the invention address technical challenges in order toimprove the efficiency of OTEC operations and reduce the cost ofconstruction and operation.

The vessel or platform 210 requires low motions to minimize dynamicforces between the cold water pipe 217 and the vessel or platform 210and to provide a benign operating environment for the OTEC equipment inthe platform or vessel. The vessel or platform 210 should also supportcold and warm water inlet (218 and 212) volume flows, bringing insufficient cold and warm water at appropriate levels to ensure OTECprocess efficiency. The vessel or platform 210 should also enable coldand warm water discharge via cold and warm water outlets (221 and 215)well below the waterline of vessel or platform 210 to avoid thermalrecirculation into the ocean surface layer. Additionally, the vessel orplatform 210 should survive heavy weather without disrupting powergenerating operations.

The OTEC heat engine 10 should utilize a highly efficient thermal cyclefor maximum efficiency and power production. Heat transfer in boilingand condensing processes, as well as the heat exchanger materials anddesign, limit the amount of energy that can be extracted from each poundof warm seawater. The heat exchangers used in the evaporator 214 and thecondenser 220 require high volumes of warm and cold water flow with lowhead loss to minimize parasitic loads. The heat exchangers also requirehigh coefficients of heat transfer to enhance efficiency The heatexchangers can incorporate material and design that may be tailored tothe warm and cold water inlet temperatures to enhance efficiency. Theheat exchanger design should use a simple construction method withminimal amounts of material to reduce cost and volume.

Turbo generators 216 should be highly efficient with minimal internallosses and may also be tailored to the working fluid to enhanceefficiency

FIG. 3 illustrates an implementation of the present invention thatenhances the efficiency of previous OTEC power plants and overcomes manyof the technical challenges associated therewith. This implementationcomprises a spar for the vessel or platform, with heat exchangers andassociated warm and cold water piping integral to the spar.

OTEC Spar 310 houses an integral multi-stage heat exchange system foruse with an OTEC power generation plant. Spar 310 includes a submergedportion 311 below waterline 305. Submerged portion 311 comprises warmwater intake portion 340, evaporator portion 344, warm water dischargeportion 346, condenser portion 348, cold water intake portion 350, coldwater pipe 351, cold water discharge portion 352, machinery deck portion354, and deck house 360.

In operation, warm sea water of between 75° F. and 85° F. is drawnthrough warm water intake portion 340 and flows down the spar thoughstructurally integral warm water conduits not shown. Due to the highvolume water flow requirements of OTEC heat engines, the warm waterconduits direct flow to the evaporator portion 344 of between 500,000gpm and 6,000,000 gpm. Such warm water conduits have a diameter ofbetween 6 ft and 35 ft, or more. Due to this size, the warm waterconduits are vertical structural members of spar 310. Warm waterconduits can be large diameter pipes of sufficient strength tovertically support spar 310. Alternatively, the warm water conduits canbe passages integral to the construction of the spar 310.

Warm water then flows through the evaporator portion 344 which housesone or more stacked, multi-stage heat exchangers for warming a workingfluid to a vapor. The warm sea water is then discharged from spar 310via warm water discharge 346. Warm water discharge can be located ordirected via a warm water discharge pipe to a depth at or close to anocean thermal layer that is approximately the same temperature as thewarm water discharge temperature to minimize environmental impacts. Thewarm water discharge can be directed to a sufficient depth to ensure nothermal recirculation with either the warm water intake or cold waterintake.

Cold sea water is drawn from a depth of between 2500 and 4200 ft, ormore, at a temperature of approximately 40° F., via cold water pipe 351.The cold sea water enters spar 310 via cold water intake portion 350.Due to the high volume water flow requirements of OTEC heat engines, thecold sea water conduits direct flow to the condenser portion 348 ofbetween 500,000 gpm and 3,500,000 gpm. Such cold sea water conduits havea diameter of between 6 ft and 35 ft, or more. Due to this size, thecold sea water conduits are vertical structural members of spar 310.Cold water conduits can be large diameter pipes of sufficient strengthto vertically support spar 310. Alternatively, the cold water conduitscan be passages integral to the construction of the spar 310.

Cold sea water then flows upward to stacked multi-stage condenserportion 348, where the cold sea water cools a working fluid to a liquid.The cold sea water is then discharged from spar 310 via cold sea waterdischarge 352. Cold water discharge can be located or directed via acold sea water discharge pipe to depth at or close to an ocean thermallayer that is approximately the same temperature as the cold sea waterdischarge temperature. The cold water discharge can be directed to asufficient depth to ensure no thermal recirculation with either the warmwater intake or cold water intake.

Machinery deck portion 354 can be positioned vertically between theevaporator portion 344 and the condenser portion 348. Positioningmachinery deck portion 354 beneath evaporator portion 344 allows nearlystraight line warm water flow from intake, through the multi-stageevaporators, and to discharge. Positioning machinery deck portion 354above condenser portion 348 allows nearly straight line cold water flowfrom intake, through the multi-stage condensers, and to discharge.Machinery deck portion 354 includes turbo-generators 356. In operationwarm working fluid heated to a vapor from evaporator portion 344 flowsto one or more turbo generators 356. The working fluid expands in turbogenerator 356 thereby driving a turbine for the production of electricalpower. The working fluid then flows to condenser portion 348 where it iscooled to a liquid and pumped to evaporator portion 344.

The performance of heat exchangers is affected by the availabletemperature difference between the fluids as well as the heat transfercoefficient at the surfaces of the heat exchanger. The heat transfercoefficient generally varies with the velocity of the fluid across theheat transfer surfaces. Higher fluid velocities require higher pumpingpower, thereby reducing the net efficiency of the plant. A hybridcascading multi-stage heat exchange system facilitates lower fluidvelocities and greater plant efficiencies. The stacked hybrid cascadeheat exchange design also facilitates lower pressure drops through theheat exchanger. And the vertical plant design facilitates lower pressuredrop across the whole system. A hybrid cascading multi-stage heatexchange system is described in U.S. patent application Ser. No. ______,(Attorney Docket No. 2556-0004001), entitled “Ocean Thermal EnergyConversion Plant,” filed on Jan. 21, 2010 and concurrently with thepresent application, the entire contents of which are incorporatedherein by reference.

As described above, OTEC operations require a source of cold water at aconstant temperature. Variations in the cooling water can greatlyinfluence the overall efficiency of the OTEC power plant. As such, waterat approximately 40° F. is drawn from depths of between 2000 ft and 4200ft or more. A long intake pipe is needed to draw this cold water towardthe surface and into the OTEC power plant.

Such cold water pipes have been an obstacle to commercially viable OTECoperations because of the cost in constructing a pipe of suitableperformance and durability. OTEC requires large volumes of water atdesired temperatures in order to ensure maximum efficiency in generatingelectrical power. Previous cold water pipe designs specific to OTECoperations have included a sectional construction. Cylindrical pipesections were bolted or mechanically joined together in series until asufficient length was achieved. Pipe sections were assembled near theplant facility and the fully constructed pipe was then upended andinstalled. This approach had significant drawbacks including stress andfatigue at the connection points between pipe sections. Moreover, theconnection hardware added to the overall pipe weight, furthercomplicating the stress and fatigue considerations at the pipe sectionconnections and the connection between the fully assembled CWP and theOTEC platform or vessel.

The cold water pipe (“CWP”) is used to draw water from the cold waterreservoir at an ocean depth of between 2000 ft and 4200 ft or more. Thecold water is used to cool and condense to a liquid the vaporous workingfluid emerging from the power plant turbine. The CWP and its connectionto the vessel or platform are configured to withstand the static anddynamic loads imposed by the pipe weight, the relative motions of thepipe and platform when subjected to wave and current loads of up to100-year-storm severity, and the collapsing load induced by the waterpump suction. The CWP is sized to handle the required water flow withlow drag loss, and is made of a material that is durable and corrosionresistant in sea water. The material and physical construction of theCWP can at least partially thermally insulate the cold water as it movesfrom depth to the OTEC plant.

The cold water pipe length is defined by the need to draw water from adepth where the temperature is approximately 40° F. The CWP length canbe between 2000 feet and 4000 ft or more. In aspects of the presentinvention the cold water pipe can be approximately 3000 feet in length.

The CWP diameter is determined by the power plant size and water flowrequirements. The water flow rate through the pipe is determined by thedesired power output and OTEC power plant efficiency. The CWP can carrycold water to the cold water conduit of the vessel or platform at a rateof between 500,000 gpm and 3,500,000 gpm, or more. Cold water pipediameters can be between 6 feet and 35 feet or more. In aspects of thepresent invention, the CWP diameter is approximately 31 feet indiameter.

Referring to FIG. 4 a continuous offset staved cold water pipe is shown.The cold water pipe 451 is free of sectional joints as in previous CWPdesigns, instead utilizing an offset stave construction. CWP 451includes a top end portion 452 for connection to the submerged portionof the floating OTEC platform 411. Opposite top end portion 452 isbottom portion 454, which can include a ballast system, an anchoringsystem, and/or an intake screen.

CWP 451 comprises a plurality of offset staves constructed to form acylinder. In an aspect the plurality of offset staves can includealternating multiple first staves 465 and multiple second staves 467.Each first stave includes a top edge 471 and a bottom edge 472. Eachsecond stave includes a top edge 473 and a bottom edge 474. In anaspect, second stave 467 is vertically offset from an adjacent firststave portion 465 such that top edge 473 (of second stave portion 467)is between 3% and 97% vertically displaced from the top edge 471 (offirst stave portion 465). In further aspects, the offset betweenadjacent staves can be approximately, 5%, 10%, 15%, 20%, 25%, 30%, 35%,40%, 45%, 50% or more.

FIG. 5 illustrates a detail view of an offsetting stave pattern of anaspect of the present invention. The pattern includes multiple firststaves 465, each having a top edge portion 471, bottom edge portion 472,connected edge 480 and offset edge 478. The pattern also includesmultiple second staves 467, each having a top edge portion 473, a bottomedge portion 474, connected edge 480, and offset edge 479. In formingthe cold water pipe, first stave section 465 is joined to second stavesection 467 such that connected edge 480 is approximately 3% to 97% ofthe length of first stave section 465 when measured from the top edge471 to the bottom edge 472. In an aspect, connected edge 480 isapproximately 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90% of thelength of the stave.

It will be appreciated that in a fully constructed pipe, first stave 465can be joined to second stave 467 along connected edge 480. First stave465 can also be connected to additional staves along offset edge 478,including an additional first stave portion, an additional second staveportion, or any other stave portion. Similarly, second stave 467 can bejoined to first stave portion along connected edge 480. And second stave467 can be joined to another stave along offset edge 479, including anadditional first stave portion, an additional second stave portion, orany other stave portion.

In aspects, the connected edge 480 between the multiple first staves 465and the multiple second staves 467 can be a consistent length orpercentage of the stave length for each stave about the circumference ofthe pipe. The connected edge 480 between the multiple first staves 465and the multiple second staves 465 can be a consistent length orpercentage of the stave length for each stave along the longitudinalaxis of the cold water pipe 451. In further aspects the connected edge480 can vary in length between alternating first staves 465 and secondstaves 467.

As illustrated in FIG. 5, first stave 465 and second stave 467 have thesame dimensions. In aspects, first stave 465 can be between 30 and 130inches wide or more, 30 to 60 feet long, and between 1 and 24 inchesthick. In an aspect the stave dimensions can be approximately 80 incheswide, 40 feet long, and 4 to 12 inches thick. Alternatively, first stave465 can have a different length or width from second stave 467.

FIG. 6 illustrates a cross sectional view of cold water pipe 451 showingalternating first staves 465 and second staves 467. Each stave includesan inner surface 485 and an outer surface 486. Adjacent staves arejoined along connected surface 480. Any two connected surfaces onopposite sides of a single stave define an angle α. The angle α isdetermined by dividing 360° by the total number of staves. In an aspect,α can be between 1° and 36°. In an aspect a can be 22.5° for a 16 stavepipe or 11.25° for a 32 stave pipe.

Individual staves of cold water pipe 451 can be made from polyvinylchloride (PVC), chlorinated polyvinyl chloride (CPVC), fiber reinforcedplastic (FRP), reinforced polymer mortar (RPMP), polypropylene (PP),polyethylene (PE), cross-linked high-density polyethylene (PEX),polybutylene (PB), acrylonitrile butadiene styrene (ABS); polyurethane,polyester, fiber reinforced polyester, nylon reinforce polyester, vinylester, fiber reinforced vinyl ester, nylon reinforced vinyl ester,concrete, ceramic, or a composite of one or more thereof. Individualstaves can be molded, extruded, or pulltruded using standardmanufacturing techniques. In one aspect, individual staves arepulltruded to the desired shape and form and comprise a fiber or nylonreinforced vinyl ester. Vinyl esters are available from Ashland Chemicalof Covington, Ky.

In an aspect, staves are bonded to adjacent staves using a suitableadhesive. A flexible resin can be used to provide a flexible joint anduniform pipe performance. In aspects of the invention, staves comprisinga reinforced vinyl ester are bonded to adjacent staves using a vinylester resin. Methacrylate adhesives can also be used, such as MA560-1manufactured by Plexis Structural Adhesives of Danvers, Mass. Referringto FIGS. 7A-7C, various stave constructions are shown wherein anindividual stave 465 includes a top edge 471, a bottom edge 472 and oneor more voids 475. Void 475 can be hollow, filled with water, filledwith a resin, filled with an adhesive, or filled with a foam material,such as syntactic foam. Syntactic foam is a matrix of resin and smallglass beads. The beads can either be hollow or solid. Void 475 can befilled to influence the buoyancy of the stave and/or the cold water pipe451. FIG. 7A illustrates a single void 475. In an aspect multiple voids475 can be equally spaced along the length of the stave, as illustratedin FIG. 7B. In an aspect, one or more voids 475 can be placed toward oneend of the stave, for example toward the bottom edge 472, as illustratedin FIG. 7C.

Referring to FIG. 8, each individual stave 465 can include a top edge471, a bottom edge 472, a first longitudinal side 491 and a secondlongitudinal side 492. In an aspect, longitudinal side 491 includes ajoinery member, such as tongue 493. The joinery member can alternativelyinclude, biscuits, half-lap joints, or other joinery structures. Secondlongitudinal side 492 includes a mating joinery surface, such as groove494. In use, the first longitudinal side 491 of a first stave mates orjoins with the second longitudinal side 492 of a second stave. Thoughnot shown, joining structures, such as tongue and groove, or otherstructures can be used at the top edge 471 and the bottom edge 472 tojoin a stave to a longitudinally adjacent stave.

In aspects of the invention, first longitudinal side can include apositive snap lock connection 497 for mating engagement with secondlongitudinal side 492. Positive snap lock connections or snap lockconnections are generally described in U.S. Pat. No. 7,131,242,incorporated herein by reference in its entirety. The entire length oftongue 493 can incorporate a positive snap lock or portions of tongue493 can include a positive snap lock. Tongue 493 can include snaprivets. It will be appreciated that where tongue 493 includes a snaplocking structure, an appropriate receiving structure is provided on thesecond longitudinal side having groove 494.

FIG. 9 illustrates an exemplary positive snap lock system, wherein maleportion 970 includes collar 972. Male portion 970 mechanically engageswith receiving portion 975 with include recessed collar mount 977. Inuse, male portion 970 is inserted into receiving portion 975 such thatcollar portion 972 engages recessed collar mount 977, there by allowinginsertion of the male portion 970 but preventing its release orwithdrawal.

Positive snap locking joints between staved portions of the offsetstaved pipe can be used to mechanically lock two staved portiontogether. The positive snap lock joints can be used alone or incombination with a resin or adhesive. In an aspect, a flexible resin isused in combination with the positive snap lock joint.

FIG. 10 illustrates a cold water pipe 451 having an offset staveconstruction comprising multiple alternating first staves 465 and secondstaves 467 and further comprising a spirally wound ribbon 497 coveringat least a portion of the outer surface of cold water pipe 451. Inaspects the ribbon is continuous from the bottom portion 454 of coldwater pipe 451 to the top portion 452 of the cold water pipe 451. Inother aspects the ribbon 497 is provided only in those portions of pipe451 that experience vortex shedding due to movement of water past thecold water pipe 451. Ribbon 497 provides radial and longitudinal supportto cold water pipe 451. Ribbon 497 also prevents vibration along thecold water pipe and reduces vortex shedding due to ocean current action.

Ribbon 491 can be the same thickness and width as an individual stave ofcold water pipe 451 or can be two, three, four or more time thethickness and up to 10 times (e.g., 2, 3, 4, 5, 6, 7 8, 9 or 10 times)the width of an individual stave.

Ribbon 491 can be mounted on the outside surface of the cold water pipeso as to lay substantially flat along the outside surface. In anembodiment, ribbon 491 can protrude outwardly from the outside surfaceof cold water pipe 451 so as to form a spirally wound strake.

Ribbon 491 can be of any suitable material compatible with the materialof the multiple staves forming cold water pipe 451, including: polyvinylchloride (PVC), chlorinated polyvinyl chloride (CPVC), fiber reinforcedplastic (FRP), reinforced polymer mortar (RPMP), polypropylene (PP),polyethylene (PE), cross-linked high-density polyethylene (PEX),polybutylene (PB), acrylonitrile butadiene styrene (ABS); polyurethane,polyester, fiber reinforced polyester, vinyl ester, reinforced vinylester, concrete, ceramic, or a composite of one or more thereof. Ribbon491 can be molded, extruded, or pulltruded using standard manufacturingtechniques. In one aspect, ribbon 491 is pulltruded to the desired shapeand form and comprises a fiber or nylon reinforced vinyl ester similarto that used with the staves of cold water pipe 451. Ribbon 491 can bejoined to cold water pipe 451 using a suitable adhesive or resinincluding the resins of any of the materials above.

In some aspects, ribbon 491 is not continuous along the length of coldwater pipe 451. In some aspects, ribbon 491 is not continuous about thecircumference of cold water pipe 451. In some aspects, ribbon 491comprises vertical strips adhered to the outside surface of the coldwater pipe 451. In some aspects, where radial or other structuralsupport is required, ribbon 491 can be a circumferential support memberaround the outside surface of the cold water pipe.

Ribbon 491 can be adhesively bonded or adhered to the outside surface ofthe cold water pipe, using a suitable flexible adhesive. In an aspect,ribbon 491 can be mechanically coupled to the outside surface of coldwater pipe 451 using multiple positive snap locks.

With regard to FIG. 11, an exemplary method of assembling a cold waterpipe provides for the efficient transport and assembly of the cold waterpipe 451. Vertical cylindrical pipe sections are assembled by aligning1110 alternating first and second stave portions to have the desiredoffset as described above. The first and second stave portions are thenjoined 1120 to form a cylindrical pipe section. The offset first andsecond staves can be joined using any of a variety of joining methods.In an aspect the multiple offset first and second stave portions arejoined using a tongue and groove arrangement and a flexible adhesive. Inan aspect the multiple first and second staved portions are joined usinga mechanical positive snap lock. A combination of tongue and groove,snap lock mechanisms, and flexible adhesives can be used.

After joining 1120 the multiple first and second stave portions to forma cylindrical pipe section having offset first and second staveportions, a retaining band, inflatable sleeve or other jig can beattached 1122 to the cylindrical pipe section to provide support andstability to the pipe section. The steps of aligning 1110 and joining1120 multiple offset first and second stave portions can be repeated1124 to form any number of prefabricated cylindrical pipe sections. Itwill be appreciated that the cylindrical pipe section can beprefabricated at the OTEC plant facility or remotely and thentransported to the OTEC plant facility for additional construction toform the fully assembled cold water pipe 451.

Having assembled at least two cylindrical pipe sections having offsetstaves, an upper and lower cylindrical pipe sections are joined 1126 andthe offset staves of each pipe section are aligned. A flexible adhesivecan be applied 1130 to the butt joint of the offset staves of the upperand lower cylindrical pipe sections. The staves of the two pipe sectionscan be joined using a variety of end butt joints including biscuitjoinery. In an aspect, the offset staves of the upper and lowercylindrical pipe portions can be provided with aligning joining voidswhich in turn can be filled with a flexible adhesive.

Gaps in and joints between pipe sections or between and individualstaves can be filled 1132 with additional flexible resin. Once the twopipe sections have been joined and the resin applied where need the twopipe sections are allowed to cure 1134.

The retaining band is then removed 1136 from the lower pipe section anda spirally wound strake is attached thereto. The spirally wound strakecan be attached using adhesive bonding, mechanical bonding, for examplepositive snap locks, or a combination of the adhesive and mechanicalbonding.

In an aspect of the method of assembly, after the spiral strake isattached to the lower pipe section, the entire pipe assembly can beshifted, for example lowered, so that the previous upper pipe portionbecomes the new lower pipe portion, 1138. Then a new upper cylindricalpipe section is assembled 1140 in a similar manner as described above.That is, first and second stave portions are aligned 1142 to achieve thedesired offset. The first and second stave portions are then joined 1144to form a new cylindrical pipe section, e.g., new upper pipe section. Aspreviously mentioned, a retaining band, inflatable sleeve or other jigcan be used to provide support and stability to the cylindrical pipesection during construction of the cold water pipe 451.

Having assembled new upper pipe section 1144, the offset staves of thenew lower pipe section and the new upper pipe section are aligned anddrawn together 1146. Adhesive or flexible resin is applied 1148 to theend butt joints as described above, for example in conjunction withbiscuit joinery or with aligning joining voids. Any gaps between the newlower pipe section and the new upper pipe section or between any twostave portions can be filled 1150 with additional flexible resin. Theentire assembly can then be left to cure 1152. The retaining jig can beremoved 1154 as before and the spiral strake can be attached to the newlower portion. And as before the entire pipe assembly can be shifted toprovide for the next cylindrical pipe section. In this manner, themethod can be repeated until the desired pipe length is achieved.

It will be appreciated that joining cylindrical pipe sections havingoffset staves can be accomplished in a number of manners consistent withthe present invention. The method of joining offset staves provides fora continuous pipe without the need for bulky, heavy or interferingjoining hardware between the pipe segments. As such a continuous pipehaving nearly uniform material properties, including flexibility andrigidity, is provided.

Example:

A cold water pipe assembly is provided that facilitates on siteconstruction of a continuous, offset staved pipe of approximately 3000feet. Additionally the staved design accounts for adverse shipping andhandling loads traditionally experienced by segmented pipe construction.For example towing and upending of traditionally constructed segmentedcold water pipes imposes hazardous loads on the pipe.

Staved construction allows offsite manufacturing of multiple staves of40 to 50 ft lengths. Each stave is approximately 52 inches wide and 4 to12 inches thick. The staves can be shipped in stacks or containers tothe offshore platform and the cold water pipe can then be constructed onthe platform from the multiple staves. This eliminates the need for aseparate facility to assemble pipe sections.

The stave portions can be constructed from a nylon reinforced vinylester having a modulus of elasticity of between about 66,000 psi and165,000 psi. The stave portions can have an ultimate strength of betweenabout 15,000 psi and 45,000 psi, with a tensile strength between about15,000 psi to 45,000 psi. In an aspect, the stave portions can have anmodulus of elasticity of 150,000 psi, an ultimate strength of 30,000 psiand a yield strength of 30,000 psi, such that the installed CWP behavessimilar to a hose rather than a purely rigid pipe. This is advantageousin storm conditions as the pipe is more flexible and avoids cracking orbreaking. In an aspect, the pipe can deflect approximately two diametersfrom center at the unconnected lower end. Deflection at the unconnectedlower end should not be so great as to interfere with the mooring systemof the OTEC power plant or any other underwater systems involved inplant operations.

The cold water pipe connects to the bottom portion of the OTEC powerplant. More specifically, the cold water pipe connects using a dynamicbearing with the bottom portion of the OTEC spar of FIG. 3. Cold waterpipe connections in OTEC applications are described in Section 4.5 ofAvery & Wu, “Renewable Energy from the Ocean, a Guide to OTEC,” OxfordUniversity Press, 1994, incorporated herein by reference in itsentirety.

One of the significant advantages of using the spar buoy as the platformis that doing so results in relatively small rotations between the sparitself and the CWP even in the most severe 100-year storm conditions. Inaddition the vertical and lateral forces between the spar and the CWPare such that the downward force between the spherical ball and its seatkeeps the bearing surfaces in contact at all times. Because thisbearing, that also acts as the water seal, does not come out of contactwith its mating spherical seat there is no need to install a mechanismto hold the CWP in place vertically. This helps to simplify thespherical bearing design and also minimizes the pressure losses thatwould otherwise be caused by any additional CWP pipe restrainingstructures or hardware. The lateral forces transferred through thespherical bearing are also low enough that they can be adequatelyaccommodated without the need for vertical restraint of the CWP.

Cold water is drawn through the cold water pipe via one or more coldwater pumps such and flows via one or more cold water passages orconduits to the condenser portion of a multi-stage OTEC power plant.

Example:

Aspects of the present invention provide an integrated multi-stage OTECpower plant that will produce electricity using the temperaturedifferential between the surface water and deep ocean water in tropicaland subtropical regions. Aspects eliminate traditional piping runs forsea water by using the off-shore vessel's or platform's structure as aconduit or flow passage. Alternatively, the warm and cold sea waterpiping runs can use conduits or pipes of sufficient size and strength toprovide vertical or other structural support to the vessel or platform.These integral sea water conduit sections or passages serve asstructural members of the vessel, thereby reducing the requirements foradditional steel. As part of the integral sea water passages,multi-stage cabinet heat exchangers provides multiple stages of workingfluid evaporation without the need for external water nozzles or pipingconnections. The integrated multi-stage OTEC power plant allows the warmand cold sea water to flow in their natural directions The warm seawater flows downward through the vessel as it is cooled before beingdischarged into a cooler zone of the ocean. In a similar fashion, thecold sea water from deep in the ocean flows upward through the vessel asit is warmed before discharging into a warmer zone of the ocean. Thisarrangement avoids the need for changes in sea water flow direction andassociated pressure losses. The arrangement also reduces the pumpingenergy required.

Multi-stage cabinet heat exchangers allow for the use of a hybridcascade OTEC cycle. These stacks of heat exchangers comprise multipleheat exchanger stages or sections that have sea water passing throughthem in series to boil or condense the working fluid as appropriate. Inthe evaporator section the warm sea water passes through the first stagewhere it boils off some of the working fluid as the sea water is cooled.The warm sea water then flows down the stack into the next heatexchanger stage and boils off additional working fluid at a slightlylower pressure and temperature. This occurs sequentially through theentire stack. Each stage or section of the cabinet heat exchangersupplies working fluid vapor to a dedicated turbine that generateselectrical power. Each of the evaporator stages has a correspondingcondenser stage at the exhaust of the turbine. The cold sea water passesthrough the condenser stacks in a reverse order to the evaporators.

Referring to FIG. 10, an exemplary multi-stage OTEC heat engine 710utilizing a hybrid cascading heat exchange cycles is provided. Warm seawater is pumped from a warm sea water intake (not shown) via warm waterpump 712, discharging from the pump at approximately 1,360,000 gpm andat a temperature of approximately 79° F. All or parts of the warm waterconduit from the warm water intake to the warm water pump, and from thewarm water pump to the stacked heat exchanger cabinet can form integralstructural members of the vessel.

From the warm water pump 712, the warm sea water then enters first stageevaporator 714 where it boils a first working fluid. The warm waterexits first stage evaporator 714 at a temperature of approximately 76.8°F. and flows down to the second stage evaporator 715.

The warm water enters second stage evaporator 715 at approximately 76.8°F. where it boils a second working fluid and exits the second stageevaporator 715 at a temperature of approximately 74.5°.

The warm water flows down to the third stage evaporator 716 from thesecond stage evaporator 715, entering at a temperature of approximately74.5° F., where it boils a third working fluid. The warm water exits thethird stage evaporator 716 at a temperature of approximately 72.3° F.

The warm water then flows from the third stage evaporator 716 down tothe fourth stage evaporator 717, entering at a temperature ofapproximately 72.3° F., where it boils a fourth working fluid. The warmwater exits the fourth stage evaporator 717 at a temperature ofapproximately 70.1° F. and then discharges from the vessel. Though notshown, the discharge can be directed to a thermal layer at an oceandepth of or approximately the same temperature as the dischargetemperature of the warm sea water. Alternately, the portion of the powerplant housing the multi-stage evaporator can be located at a depthwithin the structure so that the warm water is discharged to anappropriate ocean thermal layer. In aspects, the warm water conduit fromthe fourth stage evaporator to the warm water discharge of the vesselcan be comprise structural members of the vessel.

Similarly, cold sea water is pumped from a cold sea water intake (notshown) via cold sea water pump 722, discharging from the pump atapproximately 855,003 gpm and at a temperature of approximately 40.0° F.The cold sea water is drawn from ocean depths of between approximately2700 and 4200 ft, or more. The cold water conduit carrying cold seawater from the cold water intake of the vessel to the cold water pump,and from the cold water pump to the first stage condenser can comprisein its entirety or in part structural members of the vessel.

From cold sea water pump 722, the cold sea water enters a first stagecondenser 724, where it condenses the fourth working fluid from thefourth stage boiler 717. The cold seawater exits the first stagecondenser at a temperature of approximately 43.5° F. and flows up to thesecond stage condenser 725.

The cold sea water enters the second stage condenser 725 atapproximately 43.5° F. where it condenses the third working fluid fromthird stage evaporator 716. The cold sea water exits the second stagecondenser 725 at a temperature approximately 46.9° F. and flows up tothe third stage condenser.

The cold sea water enters the third stage condenser 726 at a temperatureof approximately 46.9° F. where it condenses the second working fluidfrom second stage evaporator 715. The cold sea water exits the thirdstage condenser 726 at a temperature approximately 50.4° F.

The cold sea water then flows up from the third stage condenser 726 tothe fourth stage condenser 727, entering at a temperature ofapproximately 50.4° F. In the fourth stage condenser, the cold sea watercondenses the first working fluid from first stage evaporator 714. Thecold sea water then exits the fourth stage condenser at a temperature ofapproximately 54.0° F. and ultimately discharges from the vessel. Thecold sea water discharge can be directed to a thermal layer at an oceandepth of or approximately the same temperature as the dischargetemperature of the cold sea water. Alternately, the portion of the powerplant housing the multi-stage condenser can be located at a depth withinthe structure so that the cold sea water is discharged to an appropriateocean thermal layer.

The first working fluid enters the first stage evaporator 714 at atemperature of 56.7° F. where it is heated to a vapor with a temperatureof 74.7° F. The first working fluid then flows to first turbine 731 andthen to the fourth stage condenser 727 where the first working fluid iscondensed to a liquid with a temperature of approximately 56.5° F. Theliquid first working fluid is then pumped via first working fluid pump741 back to the first stage evaporator 714.

The second working fluid enters the second stage evaporator 715 at atemperature approximately 53.0° F. where it is heated to a vapor. Thesecond working fluid exits the second stage evaporator 715 at atemperature approximately 72.4° F. The second working fluid then flow toa second turbine 732 and then to the third stage condenser 726. Thesecond working fluid exits the third stage condenser at a temperatureapproximately 53.0° F. and flows to working fluid pump 742, which inturn pumps the second working fluid back to the second stage evaporator715.

The third working fluid enters the third stage evaporator 716 at atemperature approximately 49.5° F. where it will be heated to a vaporand exit the third stage evaporator 716 at a temperature ofapproximately 70.2° F. The third working fluid then flows to thirdturbine 733 and then to the second stage condenser 725 where the thirdworking fluid is condensed to a fluid at a temperature approximately49.5° F. The third working fluid exits the second stage condenser 725and is pumped back to the third stage evaporator 716 via third workingfluid pump 743.

The fourth working fluid enters the fourth stage evaporator 717 at atemperature of approximately 46.0° F. where it will be heated to avapor. The fourth working fluid exits the fourth stage evaporator 717 ata temperature approximately 68.0° F. and flow to a fourth turbine 734.The fourth working fluid exits fourth turbine 734 and flows to the firststage condenser 724 where it is condensed to a liquid with a temperatureapproximately 46.0° F. The fourth working fluid exits the first stagecondenser 724 and is pumped back to the fourth stage evaporator 717 viafourth working fluid pump 744.

The first turbine 731 and the fourth turbine 734 cooperatively drive afirst generator 751 and form first turbo-generator pair 761. Firstturbo-generator pair will produce approximately 25 MW of electric power.

The second turbine 732 and the third turbine 733 cooperatively drive asecond generator 752 and form second turbo-generator pair 762. Secondturbo-generator pair 762 will produce approximately 25 MW of electricpower.

The four stage hybrid cascade heat exchange cycle of FIG. 7 allows themaximum amount of energy to be extracted from the relatively lowtemperature differential between the warm sea water and the cold seawater. Moreover, all heat exchangers can directly supportturbo-generator pairs that produce electricity using the same componentturbines and generators.

It will be appreciated that multiple multi-stage hybrid cascading heatexchangers and turbo generator pairs can be incorporated into a vesselor platform design.

Example:

An offshore OTEC spar platform includes four separate power modules,each generating about 25 MWe Net at the rated design condition. Eachpower module comprises four separate power cycles or cascadingthermodynamic stages that operate at different pressure and temperaturelevels and pick up heat from the sea water system in four differentstages. The four different stages operate in series. The approximatepressure and temperature levels of the four stages at the rated designconditions (Full Load-Summer Conditions) are:

Turbine inlet Condenser Pressure/Temp. Pressure/Temp. (Psia)/(° F.)(Psia)/(° F.) 1 Stage 137.9/74.7  100.2/56.5 2″ Stage 132.5/72.4 93.7/533′ Stage 127.3/70.2  87.6/49.5 4″ Stage 122.4/68   81.9/46

The working fluid is boiled in multiple evaporators by picking up heatfrom warm sea water (WSW). Saturated vapor is separated in a vaporseparator and led to an ammonia turbine by STD schedule, seamless carbonsteel pipe. The liquid condensed in the condenser is pumped back to theevaporator by 2×100% electric motor driven constant speed feed pumps.The turbines of cycle-1 and 4 drive a common electric generator.Similarly the turbines of cycle-2 and 3 drive another common generator.In an aspect there are two generators in each plant module and a totalof 8 in the 100 MWe plant. The feed to the evaporators is controlled byfeed control valves to maintain the level in the vapor separator. Thecondenser level is controlled by cycle fluid make up control valves. Thefeed pump minimum flow is ensured by recirculation lines led to thecondenser through control valves regulated by the flow meter on the feedline.

In operation the four (4) power cycles of the modules operateindependently. Any of the cycles can be shutdown without hamperingoperation of the other cycles if needed, for example in case of a faultor for maintenance. But that will reduce the net power generation of thepower module as a whole module.

Aspects of the present invention require large volumes of seawater.There will be separate systems for handling cold and warm seawater, eachwith its pumping equipment, water ducts, piping, valves, heatexchangers, etc. Seawater is more corrosive than fresh water and allmaterials that may come in contact with it need to be selected carefullyconsidering this. The materials of construction for the major componentsof the seawater systems will be:

Large bore piping: Fiberglass Reinforced Plastic (FRP)

Large seawater ducts & chambers: Epoxy-coated carbon steel

Large bore valves: Rubber lined butterfly type

Pump impellers: Suitable bronze alloy

Unless controlled by suitable means, biological growths inside theseawater systems can cause significant loss of plant performance and cancause fouling of the heat transfer surfaces leading to lower outputsfrom the plant. This internal growth can also increase resistance towater flows causing greater pumping power requirements, lower systemflows, etc. and even complete blockages of flow paths in more severecases.

The Cold Sea Water (“CSW”) system using water drawn in from deep oceanshould have very little or no bio-fouling problems. Water in thosedepths does not receive much sunlight and lack oxygen, and so there arefewer living organisms in it. Some types of anaerobic bacteria may,however, be able to grow in it under some conditions. Shock chlorinationwill be used to combat bio-fouling.

The Warm Sea Water (“WSW”) system handling warm seawater from near thesurface will have to be protected from bio-fouling. It has been foundthat fouling rates are much lower in tropical open ocean waters suitablefor OTEC operations than in coastal waters. As a result, chemical agentscan be used to control bio-fouling in OTEC systems at very low dosesthat will be environmentally acceptable. Dosing of small amounts ofchlorine has proved to be very effective in combating bio-fouling inseawater. Dosages of chlorine at the rate of about 70 ppb for one hourper day, is quite effective in preventing growth of marine organisms.This dosage rate is only 1/20th of the environmentally safe levelstipulated by EPA. Other types of treatment (thermal shock, shockchlorination, other biocides, etc.) can be used from time to timein-between the regimes of the low dosage treatment to get rid ofchlorine-resistant organisms.

Necessary chlorine for dosing the seawater streams is generated on-boardthe plant ship by electrolysis of seawater. Electro-chlorination plantsof this type are available commercially and have been used successfullyto produce hypochlorite solution to be used for dosing. Theelectro-chlorination plant can operate continuously to fill-up storagetanks and contents of these tanks are used for the periodic dosingdescribed above.

All the seawater conduits avoid any dead pockets where sediments candeposit or organisms can settle to start a colony. Sluicing arrangementsare provided from the low points of the water ducts to blow out thedeposits that may get collected there. High points of the ducts andwater chambers are vented to allow trapped gases to escape.

The Cold Seawater (CSW) system will consist of a common deep waterintake for the plant ship, and water pumping/distribution systems, thecondensers with their associated water piping, and discharge ducts forreturning the water back to the sea. The cold water intake pipe extendsdown to a depth of more than 2700 ft, (e.g., between 2700 ft to 4200ft), where the sea water temperature is approximately a constant 40° F.Entrance to the pipe is protected by screens to stop large organismsfrom being sucked in to it. After entering the pipe, cold water flows uptowards the sea surface and is delivered to a cold well chamber near thebottom of the vessel or spar.

The CSW supply pumps, distribution ducts, condensers, etc. are locatedon the lowest level of the plant. The pumps take suction from the crossduct and send the cold water to the distribution duct system. 4×25% CSWsupply pumps are provided for each module. Each pump is independentlycircuited with inlet valves so that they can be isolated and opened upfor inspection, maintenance, etc. when required. The pumps are driven byhigh-efficiency electric motors.

The cold seawater flows through the condensers of the cycles in seriesand then the CSW effluent is discharged back to the sea. CSW flowsthrough the condenser heat exchangers of the four plant cycles in seriesin the required order. The condenser installations is arranged to allowthem to be isolated and opened up for cleaning and maintenance whenneeded.

The WSW system comprises underwater intake grills located below the seasurface, an intake plenum for conveying the incoming water to the pumps,water pumps, biocide dosing system to control fouling of the heattransfer surfaces, water straining system to prevent blockages bysuspended materials, the evaporators with their associated water piping,and discharge ducts for returning the water back to the sea.

Intake grills are provided in the outside wall of the plant modules todraw in warm water from near the sea surface. Face velocity at theintake grills is kept to less than 0.5 ft/sec. to minimize entrainmentof marine organisms. These grills also prevent entry of large floatingdebris and their clear openings are based on the maximum size of solidsthat can pass through the pumps and heat exchangers safely. Afterpassing through these grills, water enters the intake plenum locatedbehind the grills and is routed to the suctions of the WSW supply pumps.

The WSW pumps are located in two groups on opposite sides of the pumpfloor. Half of the pumps are located on each side with separate suctionconnections from the intake plenum for each group. This arrangementlimits the maximum flow rate through any portion of the intake plenum toabout 1/16th of the total flow and so reduces the friction losses in theintake system. Each of the pumps are provided with valves on inlet sidesso that they can be isolated and opened up for inspection, maintenance,etc. when required. The pumps are driven by high-efficiency electricmotors with variable frequency drives to match pump output to load.

It is necessary to control bio-fouling of the WSW system andparticularly its heat transfer surfaces, and suitable biocides will bedosed at the suction of the pumps for this.

The warm water stream may need to be strained to remove the largersuspended particles that can block the narrow passages in the heatexchangers. Large automatic filters or ‘Debris Filters’ can be used forthis if required. Suspended materials can be retained on screens andthen removed by backwashing. The backwashing effluents carrying thesuspended solids will be routed to the discharge stream of the plant tobe returned to the ocean. The exact requirements for this will bedecided during further development of the design after collection ofmore data regarding the seawater quality.

The strained warm seawater (WSW) is distributed to the evaporator heatexchangers. WSW flows through the evaporators of the four plant cyclesin series in the required order. WSW effluent from the last cycle isdischarged at a depth of approximately 175 feet or more below the seasurface. It then sinks slowly to a depth where temperature (andtherefore density) of the seawater will match that of the effluent.

Though embodiments herein have described multi-stage heat exchanger in afloating offshore vessel or platform, drawing cold water via acontinuous, offset staved cold water pipe, it will be appreciated thatother embodiments are within the scope of the invention. For example,the cold water pipe can be connected to a shore facility. The continuousoffset staved pipe can be used for other intake or discharge pipeshaving significant length to diameter ratios. The offset stavedconstruction can be incorporated into pipe sections for use intraditional segmented pipe construction. The multi-stage heat exchangerand integrated flow passages can be incorporated into shore basedfacilities including shore based OTEC facilities. Moreover, the warmwater can be warm fresh water, geo-thermally heated water, or industrialdischarge water (e.g., discharged cooling water from a nuclear powerplant or other industrial plant). The cold water can be cold freshwater. The OTEC system and components described herein can be used forelectrical energy production or in other fields of use including: saltwater desalination: water purification; deep water reclamation;aquaculture; the production of biomass or biofuels; and still otherindustries.

All references mentioned herein are incorporated by reference in theirentirety.

Other embodiments are within the scope of the following claims.

1. A pipe comprising: an elongated tubular structure having an outersurface, a top end and a bottom end, the tubular structure comprising: aplurality of first and second stave segments, each stave segment havinga top portion and a bottom portion, wherein the top portion of thesecond stave segment is offset from the top portion of the first stavedsegment.
 2. The pipe of claim 1 further comprising a strake at leastpartially wound around the pipe on the outside surface of the tubularstructure.
 3. The pipe of claim 2 wherein the strake extends from thebottom end to the top end of the tubular structure and at leastpartially covers the outer surface thereof.
 4. The pipe of claim 2wherein the strake comprises the same material as the first and secondstave segments.
 5. The pipe of claim 1 wherein each stave segmentfurther comprises a tongue on a first side and a groove on a second sidefor mating engagement with an adjacent stave segment.
 6. The pipe ofclaim 1 wherein each stave segment is between 30 feet and 90 feetbetween the bottom portion and the top portion.
 7. The pipe of claim 5wherein each stave segment is between 10 inches and 120 inches betweenthe first side and the second side.
 8. The pipe of claim 1 wherein eachstave segment is pulltruded, extruded, or molded.
 9. The pipe of claim 1wherein each stave segment comprises polyvinyl chloride (PVC),chlorinated polyvinyl chloride (CPVC), fiber reinforced plastic (FRP),reinforced polymer mortar (RPMP), polypropylene (PP), polyethylene (PE),cross-linked high-density polyethylene (PEX), polybutylene (PB),acrylonitrile butadiene styrene (ABS); polyester, fiber reinforcedpolyester, nylon reinforced polyester, vinyl ester, fiber reinforcedvinyl ester, nylon reinforced vinyl ester, concrete, ceramic, or acomposite of one or more thereof.
 10. The pipe of claim 1 wherein eachstave segment comprises at least one internal void.
 11. The pipe ofclaim 10 wherein the at least one void is filled with water,polycarbonate foam, or syntactic foam.
 12. The pipe of claim 10 whereinthe plurality of first and second stave segments are adhesively bonded.13. The pipe of claim 10 wherein the pipe forms a cold water pipe for anOTEC power plant.
 14. An offshore power generation structure comprisinga submerged portion, the submerged portion further comprising: a heatexchange portion; a power generation portion; and a cold water pipecomprising a plurality of offset first and second stave segments. 15.The offshore power generation structure of claim 14 wherein each stavesegment comprises polyvinyl chloride (PVC), chlorinated polyvinylchloride (CPVC), fiber reinforced plastic (FRP), reinforced polymermortar (RPMP), polypropylene (PP), polyethylene (PE), cross-linkedhigh-density polyethylene (PEX), polybutylene (PB), acrylonitrilebutadiene styrene (ABS); polyester, fiber reinforced polyester, vinylester, reinforced vinyl ester, concrete, ceramic, or a composite of oneor more thereof.
 16. The offshore power generation structure of claim 14wherein the first and second stave segments are adhesively bonded. 17.The offshore power generation structure of claim 14 wherein the coldwater pipe further comprises a ribbon at least partially rapping thecold water pipe.
 18. A method of forming a cold water pipe for use in anOTEC power plant, the method comprising: forming a plurality of firstand second stave segments; and adhesively bonding alternating first andsecond stave segments such that the second stave segments are offsetfrom the first stave segments to form a continuous elongated tube. 19.The method of claim 18 wherein each of the stave segments comprisespolyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), fiberreinforced plastic (FRP), reinforced polymer mortar (RPMP),polypropylene (PP), polyethylene (PE), cross-linked high-densitypolyethylene (PEX), polybutylene (PB), acrylonitrile butadiene styrene(ABS); polyester, fiber reinforced polyester, vinyl ester, reinforcedvinyl ester, concrete, ceramic, or a composite of one or more thereof.20. The method of claim 18 wherein comprises a tongue on a first sideand a groove on a second side for mating engagement with an adjacentstave segment.